I didn't see this mentioned anywhere in the product literature or new feature listings (my apologies if it's there), but I discovered something cool in CS4 last night.
While working in a DVCPROHD 720/24p sequence, I was able to set the Playback Settings > External Device to "DV: 23.976 720x480p". This enabled a realtime (with the typical DV delay, of course) preview on my external monitor, routed through a cheap miniDV camcorder. Granted, it's a downconverted DV preview, and maybe not perfect for critical color correction, but it's a cheap and easy way to set up a client monitor.
I haven't tested this with any other sequence presets, but I suspect it will work for just about any of them.
EDIT: This appears to work for any preset; it'll match the framerate of the DV stream to the framerate of the sequence. Doesn't seem to care about the video format, either.
I thought maybe this was something in CS3 that I missed, but I just checked and it isn't there. Very nice.